1. Technical Field
This invention relates to an antenna; and more particularly relates to a multibeam antenna.
2. Description of Related Art
FIG. 1A shows an antenna 20 of U.S. Pat. No. 5,589,843 having a space tapered multi-beam antenna 24, a Butler-matrix feed network 28, and a radio receiver and/or transmitter 37. The antenna 20 is known as a space tapered one hundred twenty degree antennas having four thirty degree beams.
The radio receiver and/or transmitter 37 receives and/or provides radio receiver and/or transmitter signals from or to the 4-way Butler matrix feed network 28 via cabling 41. The radio receiver/transmitter equipment 37 is generally shown since the specific type of equipment used in an actual installation can vary widely. The Butler matrix feed network 28 is implemented using a planar microstrip design 39 shown in FIG. 1B with no crossovers and is fabricated from a printed circuit board having a dielectric substrate made of low loss ceramic material, such as glass epoxy. In general, the Butler-matrix feed network 28 has N antenna ports 29 and N receiver/transmitter equipment ports 31, where N is equal to the number of co-linear arrays of the associated antenna. As shown, the 4-way Butler matrix feed network 28 has four antenna output ports 29 and four radio receiver/transmitter input ports 31. The standard phase shift of the 4-way Butler matrix feed network 28 is as follows:
The Butler-matrix feed network 28 is connected to the space-tapered antenna 24 with equally phased cables 35 that provide phase shifting of outgoing signals to electronically steer the radiating pattern of the space-tapered antenna 24.
In FIG. 1A, the space tapered multi-beam antenna 24 has a space-tapered array 26 (ANT1, ANT2, ANT3, ANT4) with rows of radiating elements spaced at about xc2xd xcex (i.e. wavelength), where xcex is the wavelength of the electromagnetic energy to be received or transmitted. (In practice, the spacing between adjacent co-linear arrays may actually be approximately 0.47 xcex.) The number of radiating elements in outermost rows is less than the number of radiating elements in center rows in order to suppress side-lobes distortion in the antenna signal, which is typically xe2x88x929 or xe2x88x9210 Db. The space-tapered array 26 includes four co-linear arrays of associated electromagnetic radiating elements 30. Each antenna output port 29 of the 4-way Butler matrix feed network 28 is respectively connected to a respective antenna ANT1, ANT2, ANT3, ANT4 of the co-linear array 26 by cables 35 and connectors 27 associated with each antenna array. The cables 35 are all the same length (i.e. equal phase cables) so as not to introduce any phase change with respect to the signals carried thereover relative to the other cables 35. In comparison, the cables 41 need not be equal phase cables since any phase changes introduced by these cables is not relevant to the electronic beam(s) being used.
In FIG. 1A, the outermost co-linear antenna arrays ANT1 and ANT4 each comprise two radiating elements 30, while the innermost antenna arrays ANT2 and ANT3 each comprise four radiating elements 30. These radiating elements 30 are typically dipole elements, although other types of radiating element can be used. Energy is radiated or received from these dipole elements by means of a feedstrap 43 having a centrally located connector 27. The dipole elements are spaced from each adjacent dipole element of the same array by a distance approximately equal to xcex. The feed strap includes portions 45 extending beyond the lowermost and uppermost dipole element, with the end of these portions connected to the electrically conductive back plate 47 of the antenna. Such a feed strap configuration is known in the art as a Bogner type feed (see U.S. Pat. No. 4,086,598, hereby incorporated by reference).
The phase progression for the antenna beam of the antenna shown in FIG. 1A is show in the table below:
The one hundred twenty degree antennas 20 suffer from high side-lobe levels that do not meet desired customer specifications of being below xe2x88x9210 dB from the beam peak. Also, the outer beams suffer from a drop in gain as compared to the inner beams.
FIGS. 1C, 1D, 1E show frequency plots for the antenna 20 in FIG. 1A that show these problems, including frequency plots respectively at frequencies of 1.850 giga Hertz (hereinafter xe2x80x9cGHzxe2x80x9d), 1.920 GHz and 1.990 GHZ. As a person skilled in the antenna design art would appreciate, each plot shows various plot characteristics, including four plot overlays (i.e. 1LH, 1RH, 2LH, 2RH), four beam peaks in degrees, four beamwidths in degrees, four front-to-back (hereinafter xe2x80x9cf/bxe2x80x9d) ratios in decibels (hereinafter xe2x80x9cdBxe2x80x9d) and four sidelobes in degrees and dBs. In FIGS. 1C, 1D, 1E, the various xe2x80x9ctrianglesxe2x80x9d help to indicate these various plot characteristics.
The technical problem to be solved is to provide an antenna having reduced side-lobe suppression, including a spaced-tapered antenna having outer beam signals that do not have a significant drop in gain as compared to inner beam signals.
The basic idea of the present invention is to either compress the row spacing of radiating elements in the collinear arrays of the antenna, or use phase progression cables leading from the feed system to the collinear array, or both.
The invention provides a new antenna, including a space-tapered antenna, having a collinear array of radiating elements coupled via a cable feeding system to a Butler matrix feed system. In the antenna, either the collinear array has compressed rows spaced in a range of xe2x85x9c to xc2xc of a wavelength, the cable feeding system is a phase progression cable feeding system, or both.
One 120xc2x0 space-tapered antenna has eight compressed rows spaced at xe2x85x9c wavelength for providing six 20xc2x0 degree beams with xe2x88x9210 dB side lobe suppression. The six beam antenna is unique in that it provides a way to use an 8-way Butler matrix, because in the prior art there is no 6-way Butler matrix feed system.
Another 120xc2x0 space-tapered antenna has eight compressed rows spaced at xc2xc wavelength for providing four 30xc2x0 beams with xe2x88x9215 dB side lobe suppression.
A 60xc2x0 space-tapered antenna has eight compressed rows spaced at xe2x85x9c wavelength in combination with a 22 xc2xdxc2x0 phase progression cable feeding system for providing three 20xc2x0 beams with xe2x88x9214 dB side lobe suppression.
A 90xc2x0 space-tapered antenna has eight compressed rows spaced at xc2xc wavelength in combination with a 22 xc2xdxc2x0 phase progression cable feeding system for providing three 30xc2x0 beams with xe2x88x9217 dB side lobe suppression.
A 90xc2x0 space-tapered antenna has four rows spaced at xc2xd wavelength and a 45xc2x0 phase progression cabling feeding system for providing three 30xc2x0 beams with xe2x88x9212 dB side lobe suppression. For this antenna, the phase progression shifts the beams so that a center beam is down the middle, normal to the antenna. This also reduces the number of beams by one such that the radiating pattern of the antenna includes the center beam with an equally balanced number of side beams around the center beam. The phase progression may also be achieved directly in the output of the feed network.
One advantage of the present invention includes improved side-lobe distortion suppression and reduced dropoff in gain of the outer beams as compared to the inner beams. The sidelobe distortion is reduced by about xe2x88x926 dB which translates into 4xc3x97 less side lobe distortion in the antenna signal for improved signal transmission.
These embodiments provides improved side-lobe suppression and the outer beams that do not have the gain dropoff associated with prior art space tapered antennas.